1. Field of the Invention
This invention relates to solid-state power converters, and more particularly to the inverter portion of a converter which incorporates means for power conditioning the input ac line.
2. Prior Art Background Power factor as defined in sinusoidal waveform terms=KW/KVA and characterizes the power being delivered to a load, both real and reactive. A purely resistive load will have a unity power factor, indicating a simple transfer of energy to the load resistance. If the load is partially or wholly inductive, the load power factor will be less than unity. For example, a load motor startup could reduce the power factor to 0.6 pf or less. Non-linear switching loads can also reduce the power factor. This produces a reactive component that will circulate between the load and the supply causing losses related to the magnitude of the reactive component.
Aircraft loads, while on the ground, generally include inductive components, producing a low power factor on the supply line. In addition, the ground supply output harmonic distortion may be considerable, depending on the type of loads and other factors. Thus, a power converter which converts power from a source, and in turn supplies an aircraft load, must process a low input power factor and usually an appreciable harmonic distortion in the source. This process results in a lowered converter efficiency and significant line power losses.
The methods of solving this problem include at least two approaches: One method is to insert a line conditioner in the source supply line and seek to correct the harmonic distortion and/or the input power factor in this manner. Such line conditioners are available and well known in the industry. Another method is to incorporate a means for correction of the input power factor and/or harmonic distortion into a power converter circuitry. This approach, although not so well known, has been described in the technical literature. One approach, which is credited to Dr. Robert Hoft of the University of Missouri, utilizes an active filter which is connected to the de link between the rectifier and the inverter of a converter, replacing the electrolytic capacitor. The active filter, which comprises switches, an inductor and capacitor, acts to absorb and counteract the harmonic currents. It is not known, however, to have a significant corrective effect on the input power Factor.
Similarly, no state-of-art power converters are known to incorporate a means of correcting input power factor and/or harmonic distortion without also incurring severe penalties such as instabilities in control, restrictions on output power and additional required EMI filtering.
There is thus a need for a power converter which incorporates both input power factor and harmonic distortion correction without incurring penalties, and which is efficient, reliable and low in operating cost.